Proposed features/key:prominence

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key:prominence
Status: Proposed (under way)
Proposed by: jeisenbe
Tagging: prominence=*
Applies to: node
Definition: Topographic prominence of a peak in meters
Rendered as: Not rendered
Drafted on: 2009-09-28
RFC start: 2018-09-24

Proposal

Use the tag prominence=* to specify the Topographic prominence of a natural=peak in meters. Prominence is also known as "drop" or "relative height". Prominence measures the height of a summit relative to the lowest contour line encircling it but containing no higher summit within it.

Rationale

Mount Everest area; Name and elevation of Mount Everest are blocked by South Peak
Everest or South Summit?

Topographic prominence is a objective measurement of how significant a peak is. For example, lists of tallest mountains use a minimum prominence cutoff of 100, 200 or 300 meters to define an independent peak. In general, all peaks with very high prominence (over 1500 meters) are important mountains. But not all important peaks have high prominence.

Prominence might be used to select peaks for rendering, or to select significant peaks for analysis by database users. A scheme which can work both for Denmark and Switzerland, would be: If too many peaks are present within a particular area of the map at a certain zoom level, select the ones with highest prominence. Different thresholds might be used for when to render a peak symbol only, versus render the peak with a name label or elevation.

For example, the main summit of Mount Everest and the South Peak (aka the South Summit) are very close in elevation and in location. Maps that show all peaks, or all peaks over a certain elevation, do not have a clear way to distinguish between the main summit of Everest - the tallest and most prominent mountain on Earth, and South Peak, a minor sub-peak with a prominence of only 11 meters. That is, a climber descending from the main summit need only walk 11 meters up from the col to reach South Peak.

Many peaks in the Harz Mountains
Many peaks, Harz Mountains

Another problem can be seen in the Harz mountains of Germany, where over 100 peaks can be seen at one time. A map renderer could choose to filter out the peaks under a certain elevation, but this would leave the map just as crowded in the center of the mountains, where all peaks are high elevation, and empty in the foothills, even though some of the lower peaks may be quite significant.

Filtering by prominence is a simple and objective way to solve rendering problems such as these.

The prominence of a peak is the same as its elevation if it is the highest point on a continent or island. All other peaks on the same landmass have a prominence that is lower than their elevation, found by subtracting the elevation of the lowest saddle (also called the "key col") along the ridge that connects to the next higher mountain.

This can also be determined with public domain topographic maps in many places, by checking the spot elevations listed for the peak or summit, and the elevaiton for the pass or saddle. This key saddle can be found by following the lowest contour line that fully encloses the peak, but does not enclose any higher summit.

See Key:ele for details about measuring and recording elevation data properly. For OpenStreetMap, this value should be in metres above above mean sea level as defined by the EGM96 geoid model. This elevation is usually very close to national "above sea level" systems with differences < 1 meter

DEM Calculations?

It is possible to calculate the elevation of peaks and saddles, and therefore the prominence of peaks, with a digital elevation model (DEM), for example the public domain SRTM data. It has been suggested that this is an alternative to tagging prominence in the OSM database. However, DEM elevations are usually only measured to the nearest 10 by 10 meter area. In areas of steep terrain, especially for peaks but also for saddles, spot elevations calculated from DEMs are often inaccurate by 10 to 20 meters. For example, the elevations recorded in the GNIS database for peaks in the United States are calculated from a DEM, and are usually 10 to 15 meters lower than the elevations recorded on paper topographic maps, based on professional surveys.

The accuracy of DEM data can be affected by clouds and landcover. In some areas, holes and noise in the DEM data can lead to very inaccurate prominence calculations, if a gap or mistake in the data creates a false peak or saddle. This is commonly seen in mountainous areas in the tropics.

For these reasons, prominence values should be confirmed by hand and verified with reliable public domain sources. Prominence values based off of accurate spot elevations from reliable topographic maps can be precise to the nearest meter. If no elevation is available for the key saddle, extrapolation from the nearest contour line is usually accurate within 5 meters, but survey with an accurately calibrated GPS altimeter is superior.

By encouraging mountaineering, hiking and hillwalking enthusiasts to confirm saddle and peak elevations, while adding ridgelines, this new tag will lead to improved database quality, especially in regions such as the United States where current peak data is based on inaccurate imports and saddles are often missing or lack accurate elevations.

Tagging

Graphic with three peaks on an island
Vertical arrows A, B and C show the topographic prominence of three peaks on an island. The dashed horizontal lines show the lowest contours that do not encircle higher peaks. Curved arrows point from a peak to its parent. Saddle "c" in the key saddle for Peak "C". Saddle "b" is the key saddle for Peak "B"

Use the tag prominence=* together with natural=peak and ele=*. This number, in meters, may be directly calculated by subtracting the elevation of the key saddle (or "key col") from the elevation of the peak. The elevation values should be defined using the EGM96 geoid model. However, the arithmetic will lead to the same result as long as both the peak and saddle elevation are calculated using the same baseline.

For most peaks, it's only necessary to know the elevation of the nearby saddles and peaks to find the prominence of a peak. Walk to the top of the peak and record the elevation. Look around and find any taller nearby peaks. If there is only 1 taller hill, walk down the ridge line to the lowest point between the first hill and the taller hill, then record the saddle elevation.

The prominence of the peak is the elevation of the peak minus the elevation of the key saddle.

Ideally the key elevations should be determined from official sources, such as signs stating the elevation of the summit (peak) and pass (saddle), checked with public domain topographic maps, GPS and altimetry when feasible. It is also possible to calculate prominence values using public domain elevation databases, such as SRTM, but these may be inaccurate in many areas outside of North America, Japan and Europe.

See this article for more details: An introduction to Topographic Prominence

Examples

USGS Topographic Map showing contour lines and key saddles around 3 peaks
Map showing the topographic prominence of three peaks near Great Pond Mountain. Red triangles mark the peaks. The lowest contour line encircling each peak is shown in black and the green dots mark the key saddle between the smaller peak and Great Pond Mountain. Source: Wikipedia

Mauna Kea:

  • natural=volcano
  • name=Mauna Kea
  • ele= 4205
  • prominence=4205
  • source=USGS topo maps
  • note=elevation value in meters converted from USGS topo map value of 13,796 feet

(Mauna Kea is the high point of the island of Hawaii, therefore it's prominence is equal to it's elevation. The spot elevation from the USGS is more accurate than elevations available from DEMs)

Key saddle

  • natural=saddle
  • name=Humu'ula Saddle
  • ele=2010
  • source=USGS topo maps

(The key saddle between Mauna Kea and Mauna Loa. Ideally the precise elevation should be refined on survey with a GPS altimeter)

Mauna Loa

  • natural=peak
  • name=Mauna Loa
  • ele=4170
  • prominence=2160
  • source=USGS topo maps

(The prominence of Mauna Loa is found by subtracting the elevation of the saddle named "Humu'ula", which is the low point of the ridge connecting Mauna Kea and Mauna Loa)

Pu'u Poliahu - small subpeak near summit of Mauna Kea

  • natural=peak
  • name=Pu'u Poliahu
  • ele=4155
  • prominence=85
  • source=USGS topo maps

(This small peak has an elevation only a few meters below that of Mauna Kea, but it is merely a small peak on the side of the volcano, as shown by it's low prominence. It's unnammed key saddle is nearby at elevation 4070m, according to public domain USGS topographic maps.)

Tagging Errors

Do not copy prominence data from Peaklist.org, Peakbagger.com, or Wikipedia, as these lists are not compatible with the OSM license.

Do not use copyrighted topographic maps, or maps with unclear licenses (such as Soviet topographic maps).

Maps provided by the USGS, Canada[ and some European countries are available in the public domain.

Please list the prominence in meters, not in feet or other units. Whole-number, numeric values should be used only.

Features/Pages Affected

natural=peak It is proposed that this tag, prominence=* be included on the Wiki page for natural=peak as an additional tag

natural=saddle On the page for natural=saddle it will be mentioned that the elevation of a key saddle can be used to calculate the prominence of a peak.

Some peaks are tagged as natural=volcano as well, though this is also used for the center of a volcanic vent, not necessarily the high point or summit.

Further extension

A Relation of type=peak may be used to relate a peak to its "key saddle". See the discussion page for an example. This may be a helpful for peaks where the key saddle is not nearby However, this relation is not a formal part of this proposal.

Further Discussion

Please add comments, suggestions and criticisms to the talk page.

This tag was discussed recently (September 2018) in the Tagging mailing list: https://lists.openstreetmap.org/pipermail/tagging/2018-September/

This feature was originally proposed on 2009-09-28 by user hjelmis, and has been in use since that time, though it had not been brought to a vote.